Pore Structure Characterization Of Low Permeability And Tight Sandstone Reservoirs Of Huagang Formation In Xihu Depression

Low permeability and tight reservoirs have become gradually significant in petroleum exploration and development in China.These reservoirs are characterized by tiny pore-throat,complicated structure and strong heterogeneity,exerting significant effects on reservoir qualities.Huagang formation mainly consists of low permeability and tight reservoirs with inhomogeneous reservoir qualities and pore structure.In order to analyze pore structure characteristics and their effects on reservoir qualities,by using an integrated approach,including casting thin section,x-ray,grain size analysis,high pressure mercury injection,ratecontrolled mercury injection,nuclear magnetic resonance,relative permeability analysis and well logging data,characterization and classification of reservoirs and pore structure were studied,and pore evolution and their controlling factors of various pore structure were also analyzed.(1)H3 in N-1 area and H3 and H4 in N-2 area are mainly conventional and low permeability reservoirs with relatively better pore structure.Pore structure of siltstone,muddy pebbled sandstone,conglomerate,fine sandstone,medium sandstone and coarse sandstone becomes better gradually.However,other reservoirs are mainly low permeability and tight with relatively worse pore structure which is slightly affected by lithologies.(2)Pore structure is divided into four types and eight subtypes according to surface porosity,average radius of pore-throat,irreducible water saturation,granularity,quartz content and clay content.Type I and type II are mainly distributed in coarse sandstone and medium sandstone in H3 in N-1 area and H3 and H4 in N-2 area.Type III and type IV are distributed in H5 and H6 and siltstone,muddy pebbled sandstone and conglomerate in all sand groups.Moreover,reservoirs were classified into four types based on storage,permeability and fluid saturation.There is a close correspondence between reservoir types and pore structure types.(3)The pore evolution consists of three stages.The porosity loss is more than 20% due to compaction and early cements in the slow subsidence period.The porosity increment is between 3% and 10% due to dissolution and its accompanied cementations in the fast subsidence period and the uplift and erosion period.The porosity loss is between 1% and 5% due to late cements including clay and carbonate cementations in the stable subsidence period.The high-quality pore structure is mainly formed in medium and coarse sandstones in underwater distributary channel with the burial shallower than 4000 m.The reservoirs experience relatively weaker compaction,stronger dissolution and weaker late clay cementation.Besieds,local porosity preservation and strong clay-coatings are also able to promote high-quality pore structure.These analyses provide the geological basis for the interpretation and prediction of relatively high-quality reservoirs in Huagang formation.